This invention pertains to a method for centrifugal casting of contact lenses in open molds and to an apparatus for performing this method.
Casting of contact lenses in constantly rotating open molds was described for the first time in Czechoslovak Pat. Nos. 108,895 and 116,260 (counterpart to U.S. Pat. No. 3,408,429). The apparatus for performing this method consists of dishes which are rotated and carry molds inserted therein which are charged with a monomer mixture under the conditions of polymerization and in a stream of inert gas. This apparatus is subject to the disadvantage in that the inert gas strips volatile components of the polymerizing monomer mixture thus causing a lack of uniformity in the composition of the formed gel and, consequently, an undesirable deformation in the shape of the final product. This disadvantage is overcome in the procedure and apparatus described in Czechoslovak Pat. No. 159,359 in which molds charged with monomer mixture are coaxially arranged in continous vertical columns inserted into tubes. Each tube is provided with drived means for its constant rotation and an inlet for protective gas which streams along the column of molds disposed therein. When ultrathin lenses started to replace the massive and thick-walled soft contact lenses, insurmountable difficulties with the foregoing method were encountered. The casting of ultrathin lenses requires dosing of very small amounts of monomer mixture. It is impossible to evenly wet the entire useful casting surface up to its limiting circumferential edge in the completely inaccessible space between the grouped molds (as described in Czechoslovak Pat. No. 116,260). This problem is overcome, for example, in Czechoslovak patent application PV 1027-83, now Czechoslovak Certificate of Authorship No. 237,592, by an alternative method of casting in vertically rotating columns of molds. Here, the molds are introduced continuously into the rotating polymerization column from the top and, at the same time, the molds with polymerized lenses are gradually discharged from the bottom in contrast to charging and discharging the rotating polymerization column with molds all at once. Before introducing a mold into a column, monomer mixture is spread over the entire surface of the mold by a slow rotation of the steeply inclined mold, for example, by the method described in Czechoslovak patent application No. 590-84 now Czechoslovak Certificate of Authorship No. 247,723. In other respects, this very successful adaptation is subject to a disadvantage consisting of the necessity of carrying out the individual operations (dosing of the monomer mixture, a slow rotation of the mold in the inclined position and the polymerization itself) in individual items of equipment. The molds must be stored in magazines after performing each operation, such presenting the danger of attraction of dust particles to the strongly electrostatically charged surface of the molds. Incorporation of dust into the monomer mixture can result in a partial deterioration of the cast lenses. An even more serious shortcoming of this alternative procedure occurs with the polymerization of some very slowly polymerizing mixtures, for example, highly diluted mixtures, suitable for manufacturing highly swellable lenses. Passage of molds through the polymerization column must be slowed down to such an extent that the procedure becomes economically impractical. This problem is overcome in Czechoslovak patent application PV 1903-84 which provides for only a partial polymerization of the monomer mixture after passage of the molds through the column, the molds then being placed into magazines which do not rotate. However, this still increases the danger of contamination of the lens surface and also increases the need for manual attendance of the apparatus.